Piston pump



FIG. 1 is a right side view of a piston pump according to the present design;

FIG. 2 is a left side view thereof;

FIG. 3 is a back view thereof;

FIG. 4 is a front view thereof;

FIG. 5 is a top view thereof;

FIG. 6 is a bottom view thereof; and,

FIG. 7 is a perspective view thereof.

The broken lines are included for the purpose of illustrating portions of the piston pump that form no part of the claimed design. 

CLAIM The ornamental design for a piston pump, as shown and described. 